The operation of semiconductor bipolar devices is well documented. The trend toward greater miniaturization requires efficient use of available space when designing any device. Typically, field effect transistors (FET's) have been employed in many applications due to their small size relative to bipolar transistors. Although FET's and their associated circuits are much smaller than bipolar transistors and well suited for creating small memory cells, they do not operate as fast as bipolar transistors because of their higher inherent reactance. When designing bipolar transistors for speed, reactance should be held to a minimum.
Leakage currents degrade the performance of any semiconductor device. When these currents are low it is said the device is well isolated. Isolation, therefore, involves insulating and distancing adjacent devices thereby increasing overall circuit size. Therefore there is a design tradeoff between isolation and the size of the resulting circuitry.